Many types of folding knives are known. The folding knife was likely one of the first mechanical devices manufactured by humans. Of concern to the user of a folding knife is the requirement of opening the knife in order to use the knife for its intended purpose. While a folding knife is safer to carry than a fixed blade knife, the knife is of little use when in a folded condition.
Perhaps the best known opening assisted knife is the switch blade: a knife where the blade is spring biased to its open position and is captured in a knife case in a folded condition. Pushing a button releases the blade, allowing the knife to be ready for use. Such knifes generally lock the blade in an open condition.
Other types of spring loaded knives capture the blade longitudinally in the knife case, and the spring forces the knife blade longitudinally out of the case when desired. Again, a spring usually works directly on the blade in such a knife.
A problem with both of the aforementioned assisted-opening knives is that they are deemed illegal in many jurisdictions in the United States of America, as well as in other countries. Just what constitutes a switch blade knife is open to question in many jurisdictions, and a knife that uses a spring assist but requires some operation by the user other than simply pushing a button to release the knife blade may not be considered a switch blade, and hence, is legal in most jurisdictions in the United States of America.